Flowmeters measure various characteristics of flow within a pipe, for example by using various methods to convert pressure, temperature, and other measured parameters into flow rates or flow volumes. Accurate measurement of these characteristics can be critical, especially in industrial applications such as those in which a flowmeter is used to measure material flow in a chemical processing or petroleum refining plant. Flowmeters must therefore produce highly accurate measurements, even under extreme temperatures, flow rates, and pressures.
In addition, many industrial applications involve transport of materials that include more than one phase of matter (e.g., liquid and gas), also known as a multi-phase flow. In situations in which such a multi-phase flow is present, for example at or near a well head where liquid crude oil, natural gas, water, sand, and other materials may be present, few existing flowmeters are able to accurately measure or derive flow. Such inaccuracies can lead to problems in downstream systems and result in inaccurate valuations of the flow content and poor decisions regarding the management of the reservoir.
The use of ultrasound to measure flow is well established with many installations worldwide. A number of ultrasonic flowmeters have been developed to provide a direct measurement of flow velocity, including transit-time based systems in which one or more transmitting transducers and one or more receiving transducers are aimed towards a medium flowing through the flowmeter. An input voltage is applied to a transmitting transducer (transmitter) to cause it to transmit ultrasonic waves into the medium. These waves are received by a receiving transducer (receiver) and converted into an output voltage. The “time of flight” of the waves is determined by comparing the time at which the input voltage is applied to the time at which the output voltage is received.
The time required for an ultrasonic signal to travel against the flow (i.e., upstream), tup, is longer than that required to travel with the flow (i.e., downstream), tdn. The difference between upstream and downstream traveling times, Δt, is directly proportional to the flow velocity. Other flowmeter types have also been developed, including differential pressure, microwave, and Doppler ultrasound flowmeters.
There is an ongoing need for flowmeters with improved accuracy, and particularly for flowmeters that can accurately measure flow in multi-phase applications.